invoice-ninja: Fix error in cron container

The "cron" container has not been working correctly for some time.  No
background tasks are getting run, and this error is printed in the log
every minute:

> `Target class [db.schema] does not exist`

It turns out, this is because of the way the PHP `artisan` tool works.
It MUST be able to write to the code directory, apparently to build some
kind of cache.  There may be a way to cache the data ahead of time, but
I haven't found it yet.  For now, it seems the only way to make
Laravel-based applications run in a container is to make the container
filesystem mutable.

invoice-ninja/invoice-ninja.yaml

@@ -154,8 +154,6 @@ spec:
           while sleep 60; do php artisan schedule:run; done
         env: *env
         envFrom: *envFrom
-        securityContext:
-          readOnlyRootFilesystem: true
         volumeMounts: *mounts
       enableServiceLinks: false
       affinity: